1. Introduction
This invention relates to a solution for the formation of a black oxide layer over various metals, and more particularly, to the formation of a black oxide layer over copper that is especially useful for increasing the adhesion of copper to a dielectric material in the fabrication of multilayer printed circuit boards.
2. Description of the Prior Art
Multilayer printed circuit boards provide good packaging density, short conductor lengths and good reliability. At the same time, weight and space are conserved. Thus, multilayer printed circuit boards have come into increasing use during the last several years where the demand for more compact electrical apparatus has increased rapidly.
A multilayer printed circuit board is typically built from a desired number of insulating base materials clad on both sides with a thin, unpatterned copper layer. A positive photoresist is generally applied over the copper layers and exposed and developed to yield a relief resist image over the copper layer. The copper is etched with a suitable etchant whereby copper bared by development of the photoresist is removed and copper beneath the resist layers is protected from etchant resulting in copper circuits beneath the protective resist layer. The resist is removed and plastic layers or layers of a fiber material impregnated with partially cured or hardened plastic (prepegs) are placed between the conductive copper patterns of multiple innerlayers. Often the outer layer is not etched at this stage and consists of an unpatterned copper layer. The whole stack is pressed under heat and pressure to form a multilayer printed circuit board. Then, holes in a desired number and pattern are made in the multilayer printed circuit board and finally, using art accepted practices, the two outermost unpatterned copper layers are provided with conductive patterns by a process comprising application of a resist, exposing and etching. The necessary electrical connections between the different conductive layers is achieved through the plated through holes.
Processes for the formation of multilayer printed circuit boards are well known in the art and described in numerous publications including, for example, U.S. Pat. Nos. 4,075,757; 4,150,421; and 4,211,603 and in more detail by Coombs, Printed Circuits Handbook, Second Edition, McGraw-Hill Book Company, New York, Section 6 comprising pages 20-3 through 23-19, 1979; all incorporated herein by reference.
It is known in the art that there is a tendency for various layers of a multilayer printed circuit board to delaminate. Frequently, delamination occurs as a consequence of insufficient adhesion between the conductive copper patterns and the applied intermediate dielectric material. This is due to th normal smoothness of the copper conductors not providing a sufficient number of anchor points or sites for adhesion to the dielectric coating. To improve adhesion of copper to the dielectric materials, different chemical treatments of the conductive patterns have been used. One such treatment is black oxidizing of the copper conductors of the innerlayer to roughen their surface and increase their adhesion to the dielectric. Black oxidizing has met with moderate but unpredictable success.
In the aforesaid U.S. Pat. No. 4,075,757, it is recognized that there are problems attendant to the use of black oxides of copper to improve adhesion and in said patent, a roughened adhesion promoting metal layer over the conductive copper layer is substituted for the black oxide. This process is more cumbersome and costly than a black oxide layer.
The disadvantages attendant to the use of black oxide in the manufacture of multilayer circuit boards was also recognized by Slominski et al., Plating and Surface Finishing, Volume No. 59, June, 1982, pages 96 through 99, incorporated herein by reference. The authors recognize that adhesion between copper and a dielectric is often erratic and often unacceptable. The authors attribute this poor performance to the oxide layer being too thick, frangible, mechanically weak and lacking in homogeneity. The authors point out that others in the art have attempted to cure this problem by using thinner oxide coatings through dilution of the conventional caustic-chlorite bath used for the formation of black oxide coatings. However, these efforts have failed because a different unstable entity forms that shrinks in volume during thermal operations, again leading to delamination of the multilayer board. Finally, the authors describe their efforts to solve the black oxide problem using a spraying procedure under carefully controlled conditions to optimize the black oxide layer. The authors report that they obtained successful results, but the requirements for spray equipment and careful control of the treatment parameters make the authors' approach impractical for commercial use.
In U.S. Pat. No. 4,409,037, there is disclosed a black oxide formation characterized by an unusually high concentration of the chlorite and a chlorite to hydroxide ratio within a particular range. In accordance with the patent, the increased concentration of chlorite permits a decreased concentration of hydroxide resulting in consistent results in terms of improved adhesion.